4-methacryloyloxyethyl trimellitic acid (4-MET) and the corresponding anhydride (4-META) display excellent adhesion properties as adhesive monomers in dental adhesives, both on the tooth structure and also on metallic substrates, and have therefore found use in many dentine adhesion promoters of very different generations as well as in metal primers (Chang et al., “4-META use in dentistry: a literature review”, J. Prosthet. Dent. 87 (2002) 216-224). 4-META is very quickly hydrolyzed to 4-MET in aqueous solutions and it is assumed that 4-MET is responsible for the adhesion properties (S. Fujisawa, S. Ito, Dent. Mat. J. 18 (1999) 54-62).
However, aqueous solutions of 4-MET are not storage-stable, as the methacrylate group is hydrolytically split off. It was found that in aqueous solution, after only eight weeks' storage at 42° C., a large part of the original compound used is hydrolytically decomposed. Thus the use of 4-META and 4-MET in self-etching enamel-dentine adhesives which contain water as co-solvent is very problematic.
EP 1 681 283 A1 and corresponding U.S. Pat. No. 7,582,686, which is hereby incorporated by reference, discloses ether compounds of organic acids and anhydrides such as 4-(2-ethoxycarbonyl-allyloxy)-phthalic acid which are hydrolysis-stable and are said to be suitable for the preparation of self-etching dental adhesives. The hydrolysis stability of these compounds is, however, likewise unsatisfactory.
WO 03/035013 A1 and corresponding US 20040266906, which is hereby incorporated by reference, discloses aqueous, single-component self-etching dental adhesives with a pH of at most 2 which contain a polymerizable N-substituted alkylacrylic or acrylamide monomer which has at least one phosphonic acid or sulphonic acid group. The monomers are to be hydrolysis-stable.